does anyone know of a calculator that, given a VIX value and other parameters like expiration date, can generate a best-guess IV skew and prices for a chain of SPX options?
Huh? Given that VIX is an estimator based on prices of both calls and puts, how do you expect it to be possible to reliably imply the skew just from the VIX/expiry/ATM? Unless you make some arbitrary assumptions, it's not a well-formed problem.